If looking at his wife is an exception to the assur, could this question be edited to specify "a woman who is not his wife," and, correspondingly, "a man who is not her husband," if applicable? Otherwise it's somewhat misleading.
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SAHMar 13 '13 at 23:18

2 Answers
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There are two opinions in the medrash cited in Aryeh's answer; R' Yehuda permits women gazing upon men. His opinion is accepted by Sh'muel in B'rachos 48b. Although R' Yochanan follows R' Yosi's interpretation of the pasuk there, the gemara does not record that he added the phrase אם כן עשית בנות ישראל כזונות. In fact, R' Yochanan himself allowed women to gaze upon his handsome countenance (B'rachos 20a, Bava M'tzi'a 84a).

Although the Sefer Chasidim implies that it would be forbidden for a woman to gaze at a man (§614, "והוא הדין לאשה שלא תשמע קול איש שמכל שהאיש מוזהר אשה מוזהרת"), this is not generally accepted as halacha. Both R' Moshe Feinstein (Igros Moshe EH vol. 1, §69) and R' Ovadia Yosef יבלחט"א (Yabia Omer OC vol. 1, 6:5) rule that women are permitted to stare at men for aesthetic pleasure as long as they do not fantasize about the man or stare licentiously.

As far as the Sefer HaChinuch quoted in Aryeh's answer, it prohibits women from fantasizing about men other then their husband, but it does not prohibit them from gazing upon other men generally speaking.

[This command] applies in every place, at every time, to men, and to women — for it's forbidden even to them to think about men, except their husbands to whom all their desire should be directed: that is what proper Jewish women do.…

Rabbi Jose said to him, "If so, you've made Jewish women like prostitutes. Isn't it the case that, just as a man can't sate his eyes with a woman inappropriate for him, so may a woman not sate her eyes with a man not hers?…"

So, there are clearly sources that forbid it. The bigger issue here is whether women have the same prohibition of hirhur that men have (which I think deserves its own question). If they don't, there shouldn't be an issue at looking at men - unless, of course, the act of looking at men might cause a stumbling block for men to transgress (but even that sounds tenuous). In any case, neither the Rambam nor the Shulchan Aruch discuss women forbidden to look at men as halacha.